This invention pertains to telephone systems in general and, in particular, to control arrangements for decoupling auxiliary circuits from telephone lines to which they are coupled. The term line as herein used may refer to lines which interconnect telephone stations to central exchanges, to trunks which interconnect central exchanges and to virtually any other transmission media in which a need arises to bypass or disconnect temporarily circuits coupled or interposed in the transmission media.
By way of example only, an illustrative, exemplary disclosure of my invention herein is concerned with telephone line extender circuits. In a telephone system, the stations served by an exchange are, of course, located at various distances from the exchange. At the greater distances, the increased impedance of station lines limits the ability of the exchange to properly transmit and receive supervisory and voice signals to and from the stations without measures to overcome the incresed impedance. A common solution to this problem is to equip such lines with line extender circuits which operate to aid the line voltage supplied to the exchange or to repeat signals at some intermediate point along the line.
In order to test station lines, it is common to provide an exchange with a test frame and access network by means of which personnel may gain access to any desired line and apply tests thereto such as for continuity, foreign potential, impedance and the like. The use of line extender circuits, however, generally interferes with such tests. For example, the signal repeating extender circuits referred to above interrupt line continuity and, accordingly, preclude certain tests such as for continuity and impedance. The voltage aiding types of extender circuits do not interrupt line continuity, but interfere instead with certain tests at least to the extent of introducing false or misleading test results.
One common method employed to prevent extender circuit test interference is to arrange each circuit with a bypassing relay whose contacts connect appropriate ones of the input and output extender circuit terminals to shunt the circuit during periods of line testing. One such arrangement is illustrated in U.S. Pat. No. 3,825,698, which issued to C. W. Chambers, Jr. on July 23, 1974. In that arrangement, a signal detector detects a specific combination of voltages on the conductors of a telephone line indicating a test condition and, in response, operates a bypassing relay for the duration of the appearance of the specific voltage combination. Many variations of the bypassing relay method are known. Nevertheless, in the known prior art, each extender circuit is equipped with individual cicuitry for controlling the operation and release of its bypassing relay. This type of arrangement, although simple in that no structure for handling multiple bypass requests is required, is expensive in the provision of redundant control facilities for each line extender circuit. Thus, a need arises for a simple and inexpensive circuit to control the operation and release of a plurality of such bypassing relays and which has the ability of simultaneously processing multiple bypass requests. Furthermore, a need exists for a bypass arrangement that may be economically retrofitted to the great many extender circuits now in service which are not equipped for bypass operation and which thus present difficulties in the routine testing of the respective lines.